1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus that uses the optical scanning apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus develops an electrostatic latent image formed on a photosensitive member by a toner and transfers and fixes the developed toner image to a recording material, thereby forming an image on the recording material. To form the electrostatic latent image on the photosensitive member, the image forming apparatus uses an optical scanning apparatus. The optical scanning apparatus includes a laser light source that emits a laser beam, and a deflector such as a rotating polygon mirror that deflects the laser beam emitted by the laser light source so that the laser beam scans the surface of the photosensitive member in a predetermined direction. To control the light power of the laser beam scanning the surface of the photosensitive member to a target light power, the image forming apparatus executes APC (Automatic Power Control).
In the APC, the light power of the laser beam emitted by the laser light source is detected using an optical sensor such as a photodiode. A driving current to be supplied to the laser light source is gradually adjusted such that the detected light power of the laser beam reaches the target light power.
The APC includes initial APC executed as an initial operation for making preparations for image formation and normal APC executed during image formation. The normal APC is to control the light power of the laser beam during, for example, the period of scanning the surface of the photosensitive member. On the other hand, the initial APC is to perform control to decide the value of the driving current to be supplied to the laser light source in a non-turn-on state as an initial operation when image data is input to the image forming apparatus.
Japanese Patent Laid-Open No. 7-171995 describes the initial APC. The light emission amount of a laser light source relative to a supplied driving current changes depending on the temperature of the light-emitting element or the time-rate change of the laser light source. To prevent the laser light source from being damaged by an excessive driving current supplied to it at the time of initial APC, Japanese Patent Laid-Open No. 7-171995 discloses initial APC that increases the driving current to be supplied to the laser light source stepwise from 0, thereby controlling the laser beam to the target light power.
However, since the initial APC described in Japanese Patent Laid-Open No. 7-171995 executes the step of increasing the driving current stepwise, a problem is posed that a control time that is relatively long is necessary after the start of the initial APC until the light power of the laser light source stabilizes near the target light power, and image formation can be started.
In particular, in a multi-beam system using a plurality of laser light sources, the initial APC is performed first for a specific laser light source to be used to generate a synchronization signal (to be referred to as a BD signal hereinafter) to define the image write position. After the light power has approached the target light power, the APC is started for the remaining laser light sources. For the remaining laser light sources, the APC needs to be performed at a timing so as not to cause the laser beam deflected by a polygon mirror to expose the photosensitive member. To detect such a timing, the light power of the laser beam to be used to generate the BD signal needs to be adjusted to a light power that allows BD signal generation. That is, after the initial APC has been performed for the specific laser light source, the initial APC is performed for the remaining laser light sources. Hence, the time after the light power has been made to approach the target light power by the initial APC until image formation can be started for all laser light sources including the specific laser light source and the remaining laser light sources further prolongs as compared to the case in which a single laser light source is used. Hence, there is deemed necessary a technique of shortening the time after the start of initial APC until the light power of the laser light source approaches the target light power.